1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation for fluoroscopic examination of a subject, of the type wherein x-ray images of the examination subject are intermittently generated.
2. Description of the Prior Art
A fluoroscopic diagnostics installation wherein x-ray images of an examination subject are intermittently generated is disclosed in German OS 30 40 425. This known installation includes an x-ray tube connected to a high-voltage generator, and has an x-ray image intensifier video chain with a pick-up unit, and control logic for operating the high-voltage supply and the x-ray image intensifier video chain. The control logic operates the high-voltage supply so that the x-ray tube emits x-rays at an increased dose rate, however, the x-ray tube output is pulsed by the control logic, so that the overall dose to which the examination subject is exposed is reduced. Intermittent x-ray images are thereby generated, which are sampled by the video pick-up unit and are stored in an image memory. The current image stored in the image memory is then reproduced with standard video frequency until a new x-ray pulse causes the generation of a new image after, for example, five clock pulses. The new image is then overwritten into the image memory. The dose per individual image is thus maintained constant but, for example, only every fifth individual image is used to generate a displayed image, with the scan beam of the video pick-up tune being disenabled in the interim. Particularly given low pulse frequencies, this results in a stroboscopic effect in the displayed image, however, this effect can be accepted in view of the advantage of the reduced dose. In order to achieve a flicker-free reproduction of the images, the image memory can be switched to a so-called "gap filling" mode, wherein the current content of the image memory is continuously read out.
Grid-controlled x-ray tubes can be used for generating pulsed x-rays, however, such x-ray tubes are substantially more costly than conventional x-ray tubes. In a technique known as primary pulsing, the high-voltage supply for the x-ray tube is operated in a pulsed manner, however, in order to achieve sufficiently steep leading and trailing pulse edges, extremely rapid rise and decay times must be achieved, which are difficult to obtain in the context of the high-voltages of the type needed for operation of the x-ray tube. This results in a complicated overall structure of the x-ray generator. Both of the above techniques for intermittent fluoroscopy, however, have the disadvantage that the x-ray tube is operated with a high filament current, or a high anode current, which has a deteriorating effect on the useful life of the tubes.
An article in the periodical "Electromedica" (5/75), at pages 148-157 discusses the use of the integrating effect of video pick-up tubes for the purpose of producing individual x-ray exposures. After the injection of a bolus of contrast agent into an examination subject, and using an unchanging dose rate, the beam current of a video pick-up tube is interrupted for half a second, so that the light incident on the video tube target can be correspondingly integrated. The image is subsequently read out and stored, so that the stored picture can be reproduced immediately thereafter. The long integration time serves the purpose, for example, of ensuring that the complete information corresponding to the entire course of a contrast agent bolus is included in the integrated image. This technique and circuitry, however, cannot be employed in a fluoroscopic mode.
An x-ray diagnostics installation is disclosed in German OS 38 42 649 having a solid-state image pick-up unit for producing static x-ray shadowgraphs. Read-out of the signals is prevented during irradiation of the subject by x-rays with a normal dose rate, so that an integration ensues. Subsequently, the x-ray generator is switched off, and the optically stored x-ray images are transferred into an electronic memory. Again, however, this known apparatus only serves the purpose of generating a static exposure, and cannot be employed for fluoroscopy.